An Experimental Study On Friction Properties Of Oceanic Basalt Under Hydrothermal Conditions

Large subduction earthquakes(Mw>8.0)usually occur on the thrust faults at the plate interface,and ground shaking and tsunamis associated with such earthquakes have caused great disasters.Due to the insufficient data from previous researches on ocean bedrock,the conclusion tends to oversimplified: it is roughly believed that the friction coefficient of these rocks is large,thus the fault is prone to unstable slip.For this background,more experiments are needed to better understand the mechanism that controls the faulting behavior.As the main constituent of the oceanic crust,the property of basalt is the basis for better understanding the mechanism of mechanical behavior such as large earthquakes at the thrusting plate boundary.Previous experimental studies have found that under the condition of effective positive pressure of 50 MPa and fluid pressure of 100 MPa,the oceanic crust basalt obtained by the International Ocean Discovery Program(IODP)Expedition 349 exhibits unstable frictional sliding behavior indicated by velocity weakening and periodic stick-slip events at 300-600 °C(Zhang et al.,2017).In order to study whether a small amount of olivine minerals affects the friction properties of basalt,the basalt core samples collected from the International Ocean Discovery Program(IODP)Expedition 349 drilling were also used,where the new sample is different to that used in the previous experiments and contains olivine.The temperature range for the new experiments is 100-600 °C,with effective normal stress and pore water pressure of150 MPa and 100 MPa,respectively.The shearing rate was stepped within 3 orders of magnitude to acquire the velocity dependence of friction,and the mechanical data were systematically analyzed in combination with microstructural observations.In order to give a more comprehensive analysis of the experimental results,a suite of friction experiments of pure olivine was also carried out.The experimental conditions were the same as in basalt,under effective positive pressure of 150 MPa and the pore water pressure of 100 MPa,six runs of experiments were carried out in the temperature range of 100-600 °C,with the same rate stepping as used in the basalt sample to acquire,the velocity dependence of friction.Data analysis was also combined with the microscopic structural observations to understand the influence of olivine on the frictional properties of basalt.By combining the experimental data with microscopic observations,we try to know:(1)Whether the olivine-bearing basalt in this work shows the similar unstable friction sliding behavior of a basalt sample in a previous study as indicated by velocity weakening and periodic stick-slip events at 300-600 °C,or will it be different.(2)If the obtained friction properties are similar or identical,compare the experimental results of pure olivine and analyze the reasons why the influence of olivine on its results is not prominent or has no effect.(3)If the new friction properties are different from the results of the previous study,then the existence of olivine is considered to be the most likely critical factor,and the mechanism of the existence of olivine affecting the frictional properties of basalt will be discussed by comparing with the experimental results of pure olivine.After analyzing the friction experiment results of basalt samples at 150 MPa effective normal stress,100 MPa pore water pressure,and temperatures of 100-600 °C,we found that the presence of olivine does significantly affect the frictional properties of basalt,manifested by a significant slip weakening phenomenon in the high temperature range(T> 400 °C),which did not attain the steady state value in the limited displacement in our experiments,with the friction coefficient decreasing persistently with the shear displacement from 0.7 to 0.55.According to the preliminary microscopic observation,it is speculated that olivine plays a catalytic role in the friction experiment to generate a weak mineral;and the commonly accepted understanding is that the formation of a certain content of weak minerals will affect the friction strength of the fault gouge,but the content of this weak mineral generated by our experiment is obviously insufficient.So whether the weak minerals are pervasively generated needs to be further studied by detailed microscopy.In the subsequent friction experiment of pure olivine,it was found that in the late stage of the runs at 500 °C and 600 °C,there was also a phenomenon of displacement weakening,and there was serpentine formed during the deformation.After observing the flakes of both samples at the same time,it was found that with the increase of temperature,the shear structure of the deformed gouge changed from localized shear to diffusely distributed shear deformation,along with significant decrease in porosity;thus,we believe that the displacement weakening of shear strength may be related to the pressure solution process between particles in the pore fluid.